Investigations have been conducted on the potential interaction of diagnostic ultrasound with fetal monkey tissues, with studies focusing on safety as it relates to human obstetrics procedures. These studies have shown (1) second and third trimester fetal tissue temperature can be elevated approximately 1{C during clinical exposure to ultrasound, with the likelihood for an increase in heating with greater lengths of exposure; (2) the generation of heat in fetal tissues occurs more rapidly with pulsed Doppler and triple mode exposure (2-D, pulsed and color Doppler) when compared to standard two-dimensional (B-mode) imaging; and (3) ultrasound can significantly, yet transiently, affect the fetal neutrophil lineage after frequent exposure during late gestation. Recent studies have provided evidence to suggest that frequent ultrasound exposure during the third trimester diminishes hematopoietic progenitors (colony forming unit-granulocyte-macrophage [CFU-GM]) in the fetal curculation, with a compensatory increase in the bone marrow in the early neonatal period. Further data suggests that these changes may be due to increased margination which is proposed to be the result of heat-induced cell adhesion. Although, to date, no long-term ramifications of prenatal diagnostic ultrasound exposure have been identified in either humans or primate models, any evidence which suggests the possibility of an effect must be explored, and the conditions which could enhance the likelihood of an effect must be known. The principal source of such data will be from relevant animal models such as the monkey. *KEY*Sonography, Fetus, Hematopoiesis, Neutropenia, Tissue heating